This invention relates to overcurrent protection devices and in particular to an overcurrent relay for protecting an electrical power system and for communicating with a master unit such as an energy management computer.
Electrical power systems, such as alternating current (AC) systems, are widely used for transmission and distribution of electrical power to electrical loads. In order to protect distribution circuits, generators, transformers, and other major components of the electrical power system time overcurrent relays are used. The time overcurrent relay senses an overcurrent condition and trips a circuit breaker connected between the electrical power system and the electrical load. The time overcurrent relay monitors current in the electrical power system and compares it against a selected pickup point. When the monitored current is above the selected pickup point timing is initiated in accordance with a selected time-current characteristic curve. If the current drops below the pickup point before timing is completed, then the timing is reset. When the monitored current exceeds the pickup point after timing is completed the circuit breaker is tripped.
In the prior art it has been known to use a microprocessor based time overcurrent relay. The microprocessor stores in memory the selected pickup point and the time-current characteristic curves. The microprocessor converts the monitored current to a digital value and compares the digital value against the selected pickup point. When the digital value exceeds the pickup point for a preselected period of time the circuit breaker is tripped. Although this process is acceptable, it would be advantageous to process the monitored current in a different manner to obtain more accurate results. For example, the monitored current, which is an analog signal, could be periodically sampled and converted into a series of digital signals. A digital value representative of a square root of the series of digital values may be derived. This derived digital value may be processed over time to determine a processed value which is a function of both the monitored current and time. The circuit breaker is tripped in the event that the processed value is not within preset limits. Additionally, it would be desirable to be able to communicate the processed value to a remote station, such as an energy management computer.